"Justin is feeling better today." Stein said Monday. "There's some still tightness. ... It's probably going to be a couple of days. But he's definitely progressing."
Upton has been dealing with a sore left oblique since he was scratched from the starting lineup before a game in Pittsburgh on July 8. He didn't play that day and then rested his oblique the following day before playing against Texas on July 10.
Upton said Sunday he thought he had turned the corner on the injury during the series in Texas and then last week in Cincinnati when he appeared in the All-Star Game for the National League, getting a hit and stealing a base.
But Upton said the oblique tightened up on him during the first four innings of Sunday's game against the Rockies on a humid afternoon. He was 0-for-2 before leaving the game before the top of the fifth inning.
The statistics from the game don't count, though, as the game was eventually postponed due to heavy rain.
Missing Upton for any stretch of time -- even if it's only a handful of games -- certainly hurts the Padres, who enter their three-game series with the Giants on a four-game winning streak.
"It's not the best thing for your ballclub, that's for sure," said San Diego interim manager Pat Murphy.
Upton is hitting .252/.331/.426 with a team-best 15 home runs and 49 RBIs in 326 at-bats. He also leads the team with 17 stolen bases.
Will Venable got the start in left field Monday with Melvin Upton Jr. starting in center field, which was the exact move the Padres made Sunday after Justin Upton left the game.
Corey Brock is a reporter for MLB.com. Keep track of @FollowThePadres on Twitter and listen to his podcast. This story was not subject to the approval of Major League Baseball or its clubs.